1. Field of the invention
The present invention relates to expandable thermoplastic polymer particles having superior processability as well as expansion and process for producing the same. More particularly, it relates to expandable thermoplastic polymer particles superior in processability and expansion in which polymers obtained by emulsion polymerization, in particular, are pelletized by an extruder and the resultant pellets are coated or impregnated with a volatile blowing agent in an aqueous suspension medium.
2. Description of the prior art
As expandable synthetic polymer particles, expandable styrene-series polymer particles are widely known, which are usually produced by suspension polymerization. In the process, polymerization and impregnation of polymer particles with a blowing agent can be performed continuously and it is therefore very effective, but particle size distribution of polymer particles obtained is not only broader than desired, but undesired particles are contained in greater amounts. Moreover, in suspension polymerization it is difficult or impossible to polymerize alpha-alkyl styrene such as alpha-methyl styrene or to copolymerize it with styrene since reactivity or copolymerizability of alpha-alkyl styrene is low specifically in a system of suspension polymerization. This problem can be solved by the adoption of emulsion polymerization. That is, polymer latex is obtained by emulsion polymerization, then subjected to coagulation, heat-treatment and drying to thus obtain polymer powder. The resulting polymer powder is pelletized by the use of an extruder to thereby produce pellets of a desired particle size. The size of the pellets so obtained is controlled to be a desired particle size by the adjustment of pelletizing conditions including the diameter of a dice and cutting. In addition, the particle size distribution is by far narrower than that of particles prepared by suspension polymerization. The pellets prepared by an extrusion are coated or impregnated with a blowing agent in an aqueous medium to thus provide expandable synthetic polymer particles.
Notwithstanding, the expandable particles of synthetic resin prepared by emulsion polymerization, pelletization and impregnation of a blowing agent markedly decrease in processability as well as expansion with the time when those are left to stand, to such an extent that those can not be supplied to the practical use in a relatively short period.